1. Field of the Invention
The present invention relates to a driver circuit, and more particularly, to a driver circuit that can switch a load such as a semiconductor light emitting element at high speed. Furthermore, the present invention can be systematized with a photoelectric converter device and a memory device.
2. Related Background Art
An explanation will be made below as for a prior art driver circuit and a prior art semiconductor light emitting element driver circuit.
FIG. 1 is a circuit diagram showing an example of conventional driver circuits and more specifically a semiconductor light emitting element driver circuit. Referring to FIG. 1, the laser diode 50 has a cathode connected to a lower potential (GND) and an anode connected to the output of a current mirror circuit. The current mirror circuit is formed of NPN transistors 19 and 20. An active element 40 is connected between the input of the current mirror circuit and the lower potential to switch on or off a laser diode. A current mirror circuit, which is formed of MOS transistors 38 and 39, is connected between the input of the above current mirror circuit and a higher potential (V.sub.CC).
Next, an operation of the driver circuit will be explained below.
Assuming that an input current I.sub.0 is needed to emit the laser element at a laser output P.sub.0 ; the input current of the current mirror circuit formed of transistors 19 and 20 is I.sub.1 ; and the current mirror ratio is 1/n, when the switching active element 40 is turned off, a current nI.sub.1 flows through the laser diode 50. If nI.sub.1 =I.sub.0, the laser diode 50 emits with an output of P.sub.0. When the switching active element 40 is turned on, it sinks a current I.sub.1, thus causing no current flowing through the laser diode 50. At the end, the light emitting diode 50 ceases its light emitting. Therefore, the driver circuit shown in FIG. 1 allows the laser diode 50 to perform its switching operation in response to a control signal to the switching active element 40.
However, in the conventional driver circuit, there is a disadvantage in that the large capacitance being parasitic on the current mirror circuit disturbs the switching operation at high speed of the laser diode 50.
Moreover, there is a disadvantage in that when the conventional driver circuit is operated on a power source voltage of 5 volts while the voltage V.sub.L across the load is, for example, 1.5 to 2.5 volts, the current mirror circuit formed of PMOS transistors 38 and 39 cannot hold a sufficient operational margin because of the voltage V.sub.L and the base to emitter voltage of the NPN transistors 19 and 20.